The Messenger salla Allahi alayhi wa sallam said: (There is no heart except that it has a cloud like the clouds of the moon: while the moon is shining, it gets covered with a cloud and it gets dark; but when the cloud leaves it, the moon goes back to its shine)

Reported by al-Tabarani in al-Awsat and Abu Naim in al-Hulya. It is declared to be hasan by al-Albani in al-Silsilah al-Sahihah